1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine.
2. Description of the Related Art
Conventionally, an air-fuel ratio control apparatus has been known which performs air-fuel ratio feedback control using a sensor output from a gas sensor provided in an exhaust passage of an internal combustion engine.
This air-fuel ratio control apparatus detects the concentration of a specific gas (e.g., oxygen) contained in exhaust gas using a gas sensor, and performs feedback control of the air-fuel ratio on the basis of the detected concentration.
Immediately after having been started at a low temperature, an internal combustion engine may enter a start state in which condensed water is present in the exhaust passage (hereinafter also referred to as a “low-temperature start state”). In the case where a gas sensor with a heater is used in an internal combustion engine in such a low-temperature start state, condensed water may adhere to a hot sensor element heated by the heater. In such a case, the sensor element may break due to thermal shock.
An air-fuel ratio control apparatus which overcomes such a drawback has been proposed (Patent Document 1). When an internal combustion engine is in a low-temperature start state, the proposed air-fuel ratio control apparatus first performs feedback control using a sensor output from a gas sensor provided downstream of a catalyst. When the engine enters an ordinary operation state in which condensed water is not present in the exhaust passage, the apparatus switches to feedback control using a sensor output from a gas sensor provided upstream of the catalyst.
Notably, in such a low-temperature start state, a time which has elapsed before condensed water from the engine (exhaust port) discharges is longer than a time which has elapsed before the condensed water accumulated in the exhaust pipe evaporates. Also, the condensed water discharged from the engine is dispersed (scattered) when passing through the catalyst. Therefore, as compared with the upstream gas sensor, the downstream gas sensor is less likely to suffer adhesion of condensed water discharged from the engine (hereinafter referred to as “water adhesion”).
Namely, the heater of the gas sensor located on the downstream side of the catalyst can be activated earlier than the heater of the gas sensor located on the upstream side of the catalyst.
Accordingly, by performing feedback control using the sensor output from the gas sensor on the downstream side of the catalyst as in the above-described conventional air-fuel ratio control apparatus, the time between startup of the internal combustion engine and the time at which the feedback control is started can be shortened as compared with the case where the feedback control is performed using only the sensor output from the gas sensor on the upstream side of the catalyst.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2009-079546
3. Problem to be Solved by the Invention
The above-described conventional air-fuel ratio control apparatus has the following problem. There is a possibility of water adhesion even on the gas sensor downstream of the catalyst during a period between the startup of the internal combustion engine and a point in time when the temperature of the exhaust system reaches a moisture evaporation temperature. Accordingly, before that point in time, the heater of the gas sensor on the downstream side of the catalyst cannot be activated, and the feedback control of the air-fuel ratio using the gas sensor on the downstream side of the catalyst cannot be performed.
Namely, in the waiting period before the temperature of the exhaust system reaches the moisture evaporation temperature, it is necessary to control the air-fuel ratio by open control in which no sensor output is used, rather than by feedback control. Thus, control of the the air-fuel ratio in accordance with the actual change state of exhaust gas cannot be carried out.